The invention concerns a pitch derivative used to bond small coal for the purpose of transforming it into briquettes. Such briquettes are known for example from the publication of French patent no. 2446857.
Generally, bitumens are used to agglomerate small coal which when compressed into pellets renders "raw briquettes". These raw briquettes generally then undergo thermal processing at about 350.degree. C. in order to convert them into "smokeless briquettes."
The binder is required to agglomerate the pellets and to give them a certain mechanical resistance but not to the point that mixing at temperatures between approximately 80 and 95.degree. C. becomes impossible. This factor explains why technicians prefer binders that have a softening point falling between 80 and 90.degree. C. What is more, the pellets that are placed on a belt during the smokeless procedure must be resistant to crushing when they experience thermal shock as they are placed in the oven. With known binders the material obviously softens with the rise in temperature and the pellets' crush resistance correspondingly diminishes with the increase in temperature.
The goal of the invention, then, is to supply a binder which remains "discrete" up to approximately 95.degree. C. and then becomes "active" at a temperature of 100.degree. C. and remains so until the rigidity factor of carbonization takes over. It should be noted that this chemical activity must take place when the binder is dispersed in an inert medium, with coal representing approximately 90% of the mixture's mass.
A pitch derivative was described in British patent no. 1329411 which has a high softening threshold, prepared by the reaction of steam cracking tar (which contains from 30 to 50% steam cracking pitch) with an aldehyde and a third reagent, e.g., maleic anhydride. A crush resistance of between 100 and 125 daN was claimed for raw briquettes, though no details were given on their resistance at high temperatures. Furthermore, it is necessary to have the tar react with formaldehyde, along with phenol and concentrated sulfuric acid. This is inconvenient and rather expensive.
On the other hand we know from U.S. Pat. No. 4,337,193 of a composition based on tall oil pitch (from pine trees) that is modified by maleic anhydride and which contains a polyamine. However, this composition is not described as being used to bind small coal and it is impossible to deduce from this publication what the crush resistance factors are, all the more so at varying temperatures.